1. Field of the Invention
The present invention relates to a thermally stable thickener comprised of a water dispersible polyurethane and a stabilizing agent. The present invention also relates to a thermally stable thickened aqueous composition and a method of preparing the thickened aqueous composition.
2. Description of the Prior Art
It has long been known in the art to use various materials to thicken an aqueous system. Depending on the composition of the aqueous system, the gels produced with these thickeners are useful as decorative and protective coatings, paper coatings, cosmetics and personal care items, adhesives and sealants, inks, petroleum drilling fluids, packer fluids, and the like.
The thickener serves several roles in aqueous systems. In latex decorative coatings, for instance, the thickener may provide improved stability and pigment suspension, and improved application properties. In cosmetics and personal care items, the thickener will improve body, smoothness and silkiness, thereby making the product more aesthetically pleasing In petroleum drilling fluids, the thickener improves the suspension of the cuttings, thereby increasing the efficiency with which they can be removed.
Many thickeners, both natural and synthetic, are known in the art. However, all suffer from some disadvantage that limits their usage. Natural thickeners, for instance, include casein, alginates, gum tragacanth, and modified cellulose, including methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carbomethoxy cellulose. These natural products vary in their thickening efficiency, and provide poor flow and leveling properties. Further, they are subject to microbial attack which requires the additional presence of antimicrobial agents.
Synthetic thickeners include various acrylic polymers and maleic anhydride copolymers. Some of these are found to be pH dependent, others are hydrolytically unstable, and others are sensitive to various components normally found in aqueous coatings.
One type of synthetic thickener is a low molecular weight polyurethane characterized by at least three hydrophobic groups interconnected by hydrophilic polyether groups as disclosed in U.S. Pat. Nos. 4,079,028 and 4,155,892. The polyurethane is prepared by reacting at least one water soluble polyether polyol with at least one monofunctional hydrophobic organic compound selected from monofunctional active hydrogen compounds and organic monoisocyanates. There may also be present in the reaction mixture at least one water insoluble organic polyisocyanate or at least one polyhydric alcohol or polyhydric alcohol ether. The use of a related polyurethane in combination with a surfactant cothickener and a non-aqueous, inert organic diluent to thicken print paste is set forth in U.S. Pat. No. 4,180,491.
Another type of a urethane rheology modifier which is useful in water and organic solvent based compositions is disclosed in U.S. Pat. No. 4,298,511. The rheology modifier is the reaction product of a polyalkylene oxide, a polyfunctional material having at least 3 active hydrogens or at least 3 isocyanate groups, a diisocyanate and water. The reactants are selected such that there are no terminal hydrophobic groups in the rheology modifier.
A urethane thixotropic agent for coating compositions is set forth in U.S. Pat. No. 4,314,924. The thixotropic agent is prepared by reacting a mono-hydroxy compound with a diisocyanate to form a mono-isocyanate adduct. The adduct is then converted into a urea-urethane by reaction with a polyamine in an aprotic solvent in the presence of lithium chloride.
One drawback associated with polyurethane thickeners, as well as other of the thickeners discussed previously, is their failure to maintain the desired level of thickening after exposure to heat. When subjected to high temperatures often encountered in warehouses, vehicles, attics and other exposed and semi-exposed areas, the characteristics of the thickened composition can irreversibly be altered such that they are completely different from those originally prepared by the manufacturer.
Polyurethanes, per se, have long been known in the art for various uses ranging from fibers, to coatings, to foams. For example, in U.S. Pat. No. 3,923,926, a thixotropic polyurethane is prepared by reacting a polyurethane prepolymer having at least two terminal free isocyanate groups and a block polyether compound formed from ethylene oxide and propylene oxide and having at least two terminal secondary alcohol groups. To enhance the thixotropic property, finely divided silica or bentonite may be added.
In order to render the polyurethane insoluble in water and other types of solvents, diamines, dicarboxylic acids and other materials have been used as cross-linking agents. Illustrative of the art disclosing the use of cross-linking agents are J. M. Buist and H. Gudgeon's "Advance in Polyurethane Technology", pages 49 ff, the "Encyclopedia of Polymer Science and Technology", Volume II, pp. 55 ff and 527 and Bernard A. Dunbrow's "Polyurethanes, Second Edition", p. 28.
A further example of a cross-linked polyurethane is disclosed in U.S. Pat. No. 4,293,679. In that patent, solid, discrete, hydrophilic, cross-linked polyurethane particles are formed from a composition which comprises: (a) a water reactant; (b) a defined isocyanate capped prepolymer; (c) a water soluble solvent for (b); and (d) when the reaction functionality of (b) is two, a cross-linking agent containing at least three functional groups.
A different type of cross-linked polyester-urethane is described in U.S. Pat. No. 3,804,810. The polymer is prepared by reacting (a) a defined linear polyester, (b) at least one organic diisocyanate, and (c) at least one cross-linking agent having a functionality greater than 2. The cross-linked polyester-urethane is stated as being useful as adhesives, coating compositions, primers, and for binding magnetic and/or conductive particles to a suitable support.
Despite the activity in the art, there has not been produced a thermally stable thickener which is useful for increasing the viscosity and improving the leveling characteristics of an aqueous composition in an efficient manner which is consistent over a wide range in temperature.